1. Field of the Invention
This invention relates to nickel-cobalt base alloys and, more particularly, nickel-cobalt base alloys having excellent corrosion resistance combined with high strength and ductility at higher service temperatures.
2. Description of the Prior Art
U.S. Pat. No. 3,356,542, Smith, issued Dec. 5, 1967 (the "Smith" patent), discloses cobalt-nickel base alloys containing chromium and molybdenum. These alloys are claimed to be corrosion resistant and capable of being work-strengthened under certain temperature conditions to have very high ultimate tensile and yield strengths. These patented alloys can exist in one of two crystalline phases, depending on temperature. They are also characterized by a composition-dependent transition zone of temperatures in which transformations between phases occur. At temperatures above the upper temperature limit of the transformation zone, the alloys are stable in the face-centered cubic ("FCC") structure. At temperatures below the lower temperature of the transformation zone, the alloys are stable in the hexagonal close-packed ("HCP") form. By cold working metastable face-centered cubic material at a temperature below the lower limit of the transformation zone, some of it is transformed into the hexagonal close-packed phase which is dispersed as platelets throughout a matrix of the face-centered cubic material. It is this cold working and phase-transformation which is indicated to be responsible for the ultimate tensile and yield strengths of the patented alloys. However, the alloys of the Smith patent have stress rupture properties which make them unsuitable for temperatures above about 800.degree. F. (427.degree. C.).
U.S. Pat. No. 3,767,385, Slaney, issued Oct. 23, 1973 (the "Slaney" patent), discloses a cobalt-nickel alloy which is an improvement on the Smith patent and which has stress rupture properties suitable for service temperatures to about 1100.degree. F. (539.degree. C.). In this patent, the composition of the alloy was modified by the addition of aluminum, titanium and columbium in order to take advantage of additional precipitation hardening of the alloy, supplementing the hardening effect due to conversion of FCC to HCP phase. The alloys disclosed include elements, such as iron, in amounts which were formerly thought to result in the formation of disadvantageous topologically close-packed phases such as the sigma, mu or chi phases (depending on composition), and thus thought to severely embrittle the alloys. But this disadvantageous result is said to be avoided with the invention of the Slaney patent. For example, the alloys of the Slaney patent are reported to contain iron in amounts from 6% to 25% while being substantially free of embrittling phases.
According to the Slaney patent, it is not enough to constitute the patented alloys within the specified ranges of cobalt, nickel, iron, molybdenum, chromium, titanium, aluminum, columbium, carbon, and boron. Rather, the alloys must further have an electron vacancy number (N.sub.v), which does not exceed certain fixed values in order to avoid the formation of embrittling phases. The N.sub.v number is the average number of electron vacancies per 100 atoms of the alloy. By using such alloys, the Slaney patent states that cobalt-based alloys which are highly corrosion resistant and have excellent ultimate tensile and yield strengths can be obtained. These properties are disclosed to be imparted by formation of a platelet HCP phase in a matrix FCC phase and by precipitating compound of the formula Ni.sub.3 X, where X is titanium, aluminum and/or columbium. This is accomplished by working the alloys at a temperature below the lower temperature of a transition zone of temperatures in which transformation between HCP phase and FCC phase occurs and then heat treating between 800.degree. F. (427.degree. C.) and 1350.degree. F. (732.degree. C.) for about 4 hours.
However, none of these prior art references disclose the unique alloy of the present invention which retains excellent tensile and ductility levels and stress rupture properties at temperatures up to about 1350.degree. F. (732.degree. C.). This improvement in higher temperature properties is believed to be due to the precipitation of a stable ordered phase in addition to the higher temperature stability of the HCP phase and minimization of the topologically by close-packed (TCP) phases. Presence of these phases has deleterious effects on the mechanical properties which are well-known to those skilled in the art. The alloys of the prior art, i.e. the Slaney patent, retain their strength only up to 1100.degree. F. (593.degree. C.) and above this temperature show poor stress rupture properties.